assert in `findInstantiationOf` with concepts & constrained partial specialization

[royjacobson]

Tried to take a look at #53354, managed to hit a weird assertion:

template<typename T>
struct X {};

template <class T>
concept C = requires (T t)
{
  requires sizeof(t) > 4;
};

template <class T>
struct A{};

template <class T>
  requires C<T>
struct A<T> {};

int main() {
  A<X<int>> a;
}

Godbolt: https://godbolt.org/z/W5qc184Yn

I tried to trigger this without the sizeof but didn't manage to.
With asserts disabled it seems to compile correctly.

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[ilya-biryukov]

Review for the fix: D127487